1. Field of the Invention
The present invention relates to a discharge lamp lighting apparatus for lighting a discharge lamp, such as a high-pressure mercury lamp, and a discharge lamp apparatus where the discharge lamp lighting apparatus and a discharge lamp are combined.
2. Description of the Related Art
The discharge lamp lighting apparatus converts the input power from the power supply into AC voltage and AC current for driving the discharge lamp, and supplies the AC voltage and AC current to the discharge lamp. The discharge lamp is driven by the AC voltage and AC current, and is lit. The discharge lamp tube voltage (AC voltage) of the discharge lamp fluctuates due to dispersion or time-based changes of the discharge lamp characteristics, but in the discharge lamp lighting apparatus, constant power control is performed for maintaining the discharge lamp tube power to be constant, regardless the fluctuation of the discharge lamp tube voltage.
However if the abovementioned constant power control is performed in an area where the discharge lamp tube voltage (AC voltage) is low, the discharge lamp tube current (AC current) suddenly increases as the discharge lamp tube voltage drops. The ratio of the factors which cause the loss in the discharge lamp lighting apparatus is high in a component depending on the discharge lamp tube current, and a sudden increase in the discharge lamp tube current causes an increase of loss in the discharge lamp lighting apparatus.
Also recently discharge lamp apparatuses are designed with less margin in cooling conditions to decrease the size and weight of the device, so an increase in the loss of the discharge lamp lighting apparatus easily causes a rise in the temperature of the discharge lamp lighting apparatus. If the temperature of the discharge lamp lighting apparatus rises, the overheat protection function of the discharge lamp apparatus is activated, which stops the supply of power to the discharge lamp and turns the discharge lamp OFF.